Results and discussion

Redox potential can be used to classify soils and monitor management effects

The redox potential Eh of soils ranged from reduced to slightly oxidized +340 to +560 mV and from acidic pH 5.8 to slightly alkaline pH 7.6 (Figure 1). Most of the sampled soils were in a region of the Eh/pH space, which is considered favorable for plant growth (Husson, 2013). Compared with the favorable region, one soil was too reduced and 10 samples were too oxidized. The reduced sample was a seasonally waterlogged silty clay loam soil, the oxidized alkaline sample was a low organic matter sandy loam soil. The acidic and oxidized soils were a more complex set of soils with either a very high OM level (>12%) or a very high clay level (>60%). The two soils in the ”optimum” range were silty clays with an OM 6% and a history of grass cultivation and horse manure application. Compared to the four soils evaluated in Husson et al. (2016), the soils in this study were more reduced (530 vs. 470 mV), which could be expected on the basis of differences in climate (warm vs. cool temperate) and organic matter level (2% vs. 8%).
The addition of carbon compounds to the soil through carbon farming decreased Eh (Figure 1 and Figure 2). This supports the findings of Husson et al. (2016) where conservation agriculture (residue retention and minimum tillage) reduced Eh by 10-25 mV. In this study, the reduction was on average -20 mV (p < 0.05; 95% range -94-+26 mV). The largest reductions (-90 mV) were on sites that had compost soil amendments applied. This would suggest that soil redox potential reacts to the addition of readily decomposable organic matter. Overall, the redox potential test on dried and rewetted soils could differentiate soils based on their OM and clay status, but it also reacted on short-term changes to soil management. Poor soil condition (waterlogging, OM or tillage) was reflected by the placement in the Eh/pH chart. This reactivity to soil conditions and management makes redox a promising indicator of soil health.